How can I make a single photorealistic bird feather?
Creating the stalk thing:
To make the the stalk of the feather (or whatever it is called), create a UV sphere. Then, switch to wireframe view, go into edit mode, select the entire sphere and scale it down a bit. Next select the upper half of the sphere and move it upwards and make it smaller.

Setting up the actual feathery bits:
First, subdivide the middle bit by pressing ctrl + r and then scroll downwards a lot to get about 200 or 250 divisions. To set up for feather creation, use alt + RMB on one of inner edges . Then select the same face on the other side with shift + alt + RMB. Once you have the faces selected, create a new vertex group, and click the assign button. You can rename your vertex group if you wish by double clicking it.

I decided to remodel the tip of the feather to make it a bit longer and less spherical with proportional editing. This step is optional:

Particle system settings:
Now for the exciting bit.

Create a new particle system
Switch the particle type from emitter to hair
Enable advanced settings
Switch to vertex mode
Uncheck the box marked &quot;random&quot;
Set the density group to the vertex group we made earlier
adjust the length to something that you like


Combing:
Now for the super exciting part.

Switch to &quot;particle edit&quot; mode

Change the brush to &quot;cut&quot;

Cut away excess hair

Switch the brush to &quot;comb&quot;

Comb to satisfaction



And now you are done!
The only thing left to do is to change the hair render settings to whatever you want!
Here are the cycles thickness settings that I used:

And here are some renders:


In the macro render, you will notice that the feathers aren't quite attached to the stalk. You can fix this by adding a solidify modifier to the shaft and making it a bit thicker.